Equalization is a common term in the x-ray radiography field which refers to the process of selectively attenuating portions of the x-ray beam that are too intense for the density of a corresponding portion of the patient or object ("subject") exposed to the x-ray. Without equalization, the resulting x-ray image (appearing on either film or a monitor) may have inconsistent overall exposure, manifested as light and dark areas in the image, as a result of corresponding variations in density in the exposed portions of the subject. The effect of equalization is to reduce the intrinsically large dynamic range of the x-ray beam intensities in order to accommodate the dynamic range limitations of the x-ray detector system. The most common detector systems employed in diagnostic x-ray radiography are film and image intensifier-TV systems, both of which have severely limited dynamic range.
Equalization is also useful in digital subtraction angiography (DSA). DSA is a known imaging technique where digital radiographic images are obtained both before and after injection of an iodine based dye into the vasculature, and then the two images are subtracted. DSA employs an x-ray image intensifier (fluoroscope) that is optically coupled to a high quality television chain and to a video digitizer. Although the image intensifier has a relatively large dynamic range (i.e., the ratio of the highest allowable signal intensities to the lowest is large), the TV camera presents substantial dynamic range limitations and thus limits the dynamic range of the entire imaging system.
It is known to equalize an x-ray radiographic image by selectively attenuating only those areas of the image that are determined to have been overexposed. One such method involves arranging a plurality of filters between the x-ray emitter and the image receptor. The filters are selected and arranged so that only the areas of over-exposure are attenuated. Practice of this method provides acceptable results once the correct combination of filters has been found. However, a serious drawback of this method is that it is cumbersome since filter selection and juxtaposition is a manual process, and can require time consuming trial and error for the correct combination to be found.
It is therefore desirable to provide an apparatus and method for performing selective equalization of x-ray radiographic images that is automated and rapid, but yet is simple and relatively inexpensive to implement. The present invention achieves these goals.